1. Field of the Invention
The present invention generally relates to a wireless communication system including a plurality of mobile stations each having a wireless communication terminal installed therein, a plurality of base stations each having known positions and a communication control station which controls communication between said plurality of wireless communication terminals and said plurality of base stations. More particularly, the present invention relates to a method of determining a position of a wireless communication terminal in the above mentioned wireless communication system.
2. Description of the Related Art
In the wireless communication system of the kind mentioned above, there will be developed a number of applications utilizing a technique for determining a position of a wireless communication terminal, particularly a personal handy-phone system called PHS. Heretofore, a base station system has been proposed as a method of determining a wireless communication terminal. Recently, there has been developed GPS (Global Positioning System) system using a GPS receiver. These two systems have their own merits and demerits. Particularly, in the base station system, in order to attain a sufficiently high precision in the position detection, it is necessary to provide a larger number of base stations. It is apparent that this requires a larger cost in constructing the base stations. If a number of base stations is small, a sufficiently high precision could not be obtained.
In Japanese Patent Application Laid-open Publication Kokai Hei 7-154848, there is proposed a method of determining a position of a wireless communication terminal in the base station system, in which an electromagnetic wave transmitted by a wireless communication terminal installed in a mobile station is received by a base station whose position is known, and a distance from the mobile station to the base station is measured in accordance with a strength of a received signal, and a position of the mobile station is determined on the basis of the thus measured distance.
However, in the method of measuring a distance on the basis of a strength of a received electromagnetic wave, it is difficult to attain a sufficiently high precision for determining a position of a wireless communication terminal installed in a mobile station although it is possible to determine a base station closet to the relevant mobile station. Therefore, in this case, a position of a mobile station can be determined with a unit of communication zones of respective base stations. In order to obtain valuable position information, respective communication zones have to be sufficiently small. Then, a larger number of base stations have to be distributed with a sufficiently high density. This requires a larger cost.
In Japanese Patent Application Laid-open Publications Kokai Hei 1-206279, 3-235077 and 5-60853, there has been proposed a method of measuring a distance between a mobile station having a wireless communication terminal installed therein and a base station is measured by utilizing a propagation delay time. In this method, a signal transmitted by a mobile station is received by at least three base stations positions of which are known and which possess a substantially synchronized time for measuring, and distances from the relevant mobile station and respective base stations are measured on the basis of differences in reception times, and then a position of the mobile station on a plane is determined from the thus measured at least three distances.
In general, a position on the plane is determined by using the hyperbolic navigation. In this hyperbolic navigation, a curve on which a difference in distances measured at two base stations becomes constant is a hyperbolic curve having a foci at these two base stations. Therefore, among three base stations, one base station is selected as a common base station and two sets of hyperbolic curves are derived and a cross point of these two hyperbolic curves is determined as a position of the mobile station. In this case, a called loran diagram may be utilized, but if the base stations are not arranged in accordance with the loran diagram, it could not be utilized. Furthermore, in this known method, it is difficult to take into account of an error in the distance measurement.
It is also possible to determine a position of a mobile station by solving simultaneous ternary quadratic equations, but calculation becomes extremely complicated. The calculation may be simplified by approximating the simultaneous ternary quadratic equations by Tailor expansion. However, such an approximation is limited to a case, in which a distances between the mobile station and the base stations are extremely longer than a traveling distance of the mobile station over which a position of the mobile station must be renewed.
In the GPS method, a position could not be determined any more if electromagnetic waves from GPS satellites high above in the sky could not be received. Therefore, a position could not be determined in buildings, underground and tunnels. Moreover, it is sometimes difficult to determine a position in a city area in which many toll buildings are aligned.